Proteolysis mediated by the ubiqutin/proteasome system has emerged as a general theme in the control of the major transitions of the eukaryotic cell cycle. The SCF ubiquity ligase complex plays a central role in the regulation of the G1/S transition by mediating the proteolytic destruction of key cell cycle regulators. In fission yeast, this includes Rum 1p, an inhibitor of cyclindependant kinase (CD1k) and Cdc l8p, a highly conserved replication initiator protein. Failure to degrade these proteins results in abnormal cell cycle control characterized by excessive DNA synthesis and polyploidization. The fission yeast SCF complex contains Pop 1p and Pop2p, two highly related, but functionally non-redundant proteins with F-box/WD repeat motifs. Pop1p and Pop2p form heterooligiomeric complexes that cooperate in binding and targeting substances for proteolysis. The proposal addresses the role of Pop1p/Pop2p hetrooligogmerization in the regulation of SCF activity during the cell cycle. The study tests the hypothesis that regulated Pop1p/Pop2p hetercomplex formation during the cell cycle. The study tests the hypotheses that regulated Pop1p/Pop2p heterooligomerization creates surfaces for interactions with other proteins that are required for SCF complex formation and substrate binding. The research described proposes to (1.) determine the regulation of Pop1p/Pop2p heterocomplex formation during the cell cycle, (2.) identify domains and regulatory mechanisms involved in the Pop1p/Pop2p interaction, and (3.) isolate factors which directly interact with Pop1p/Pop2p heterooligomers and regulate their activity. SCF components are conserved from yeast to man and ubiguitin-depentdant proteolysis controls the abundance of critical cell cycle regulators in human cells. These include the Cdk inhibitors p21 and p27, the tumor suppressor p53, and G1/S cyclins, all of which have been implicated in cancer. The dramatic rearrangements of SCF components observed in human cancer cells may cause dysregulation of labile cell cycle regulators. Focusing on the biochemical details of SCF function in fission yeast may therefore reveal novel strategies to modulate its activity. In the future, this knowledge may support the design of compounds that specifically interfere with aspects of SCF functions involved in tumor development and progression.